The present invention is related to semiconductor circuit fabrication, and more particularly to a method of identifying process end points which is particularly well suited for use in real time monitoring and process control. The present invention further comprises a preferred high wavelength resolution spectrometer system for use in developing intensity vs. Wavelength spectra, which are utilized in practice of said method.
It is well known that in the fabrication of semiconductor circuitry, (e.g. integrated circuits), it is necessary to perform etching procedures. For instance, it is common to grow silicon dioxide atop a silicon substrate of a doping type, and then, utilizing photolithography techniques, open windows in said silicon dioxide, so that an opposite type dopant can be diffused into the underlying silicon. Resulting PN junctions are rectifying, and when multiple PN junctions are placed in appropriate relationship to one another, bipolar and mosfet transistors, as well as silicon controlled rectifiers etc., result. Interconnection of various such devices fabricated on a semiconductor substrate, it is noted, results in an integrated circuit.
The etching of silicon dioxide, (a very relevant example of an area of application of the present invention), can be accomplished in an etching chamber which contains fluorine or chlorine in the presence of a plasma. A reduced pressure, (e.g. 10xe2x88x925 Torr), ambient into which is introduced CF4, or more commonly, C2F6 or C4F8, gas is often utilized in industrial settings. While silicon dioxide is being etched in such a setting, certain etch products are formed, and if a beam of electromagnetic radiation is caused to pass through them, said products relatively strongly absorb energy at specific wavelengths, while energy present at other wavelengths is less strongly affected. Alternatively, energy provided by a present plasma serves to excite etch products and emissive electromagnetic radiation therefrom can be monitored. Careful monitoring of such intensity vs. wavelength spectra as a function of time can provide insight as to when silicon dioxide available for etching has been etched away, and when underlying silicon is reached. For instance, upon reaching silicon, the products of etching silicon dioxide are greatly reduced, (some small amount of said silicon dioxide etch products can still be produced as a result of typically undesirable overetching laterally under photoresist defined boundaries, however). And it is possible that new products due to interaction of plasma and etching gas with silicon will appear and affect monitored intensity vs. wavelength spectra. This is particularly true where some oxygen is present and the underlying silicon is etched. However, the products of said interaction of plasma and etching gas with silicon, it is to be understood, typically demonstrate very different electromagnetic spectrum absorbance and/or emission characteristics. It is to be understood that the procedure comprising detection of products of an etch procedure as an indication of etch end point, can be practiced where other than silicon dioxide is etched, (e.g. Al, SiN and W).
A recent paper which describes the use of low pressure high density plasma etching of silicon dioxide is titled xe2x80x9cChemical Challenge of Submicron Oxide Etchingxe2x80x9d, by McNevin et al., J. Vac. Technol. B 15(2) (March/April 1997).
Another paper is titled xe2x80x9cAn Integrated System of Optical Sensors For Plasma Monitoring And Plasma Controlxe2x80x9d, Anderson and Splichal, SPIE Vol. 2091, (1994). Real-time plasma etching utilizing sensors which measure plasma properties directly related to desired wafer features are discussed.
A paper by Splichal and Anderson titled xe2x80x9cApplication of Chemometrics to Optical Emission Spectroscopy For Plasma Monitoringxe2x80x9d, SPIE Vol. 1595, (1992) is also identified as monitoring of real-time plasma etching processes, based upon sensors which measure plasma properties that relate directly to desired etch features, is discussed.
A paper by Benson et al. titled xe2x80x9cSensor Systems For Real-time Feedback Control Of Reactive Ion Etchingxe2x80x9d, J. Vac. Sci. Technol. B 19(1), (January/February 1996), is identified as it describes use of an optical emission spectroscopy system sensor utilized in multivariant feedback control of plasma etching of wafers.
A paper titled xe2x80x9cEtchingxe2x80x940.35 m Polysilicon Gates On A High-Density Helicon Etcherxe2x80x9d, by Kroft et al., J. Vac. Sci. Technol. B 14(1) (January/February 1996), is disclosed as it describes an example of application plasmas in selective polysilicon-to-oxide plasma etching procedures.
A paper by Oh, Stanton, Anderson and Splichal titled xe2x80x9cIn Situ Diode Laser Absorption Measurements Of Plasma Species In A Gaseous Electronics Conference Reference Cell Reactorxe2x80x9d, J.Vac. Sci. Technol B 13(3) (May/June 1995). is identified as it discusses monitoring of electromagnetic absorption during etching procedures.
A paper by Manukonda and Dillon titled xe2x80x9cOptical Emission Spectroscopy of H2xe2x80x94CO and H2Oxe2x80x94CH3OH Plasmas For Diamond Growthxe2x80x9d, J.Vac. Sci. Technol. A 13(3) (May/June 1995), is identified as it describes monitoring of electromagnetic emissions during a procedure in which diamond was grown.
A paper by Litvak, titled xe2x80x9cEnd Point Control Via Optical Emission Spectroscopyxe2x80x9d, J. Vac. Sci. Technol. B 14(1) (January/February 1996) describes the use of optical emission spectroscopy in identifying oxide etch end points, utilizing a conventional monochromator/photomultiplier system in conjunction with an end-point detecting algorithm. Many additional papers which describe plasma etching in the semiconductor fabrication area exist.
Known papers which utilize Reflected Electromagnetic Radiation Intensity and Ellipsometry to investigate Etching of semiconductor systems are:
xe2x80x9cOptical Etch-Rate Monitoring Using Active Device Areas: Lateral Interference Effectsxe2x80x9d, by Heimann, J. Electrochem. Soc., Vol. 132, No. 8, (1985);
xe2x80x9cUltraviolet-Visible Ellipsometry For Process Control During The Etching Of Submicron Featuresxe2x80x9d, by Blayo et al., J. Op. Soc. Am., Vol. 12, No. 3, (1995);
xe2x80x9cMultiwavelength Ellipsometry For Real-Time Process Control Of The Plasma Etching Of Patterned Samplesxe2x80x9d, Maynard et al., J. Vac. Sci. Technol. B 15(1) (1997); and
xe2x80x9cOptical Etch Rate Monitoring: Computer Simulation Of Reflectancexe2x80x9d, Heimann et al., J. Electrochem. Soc., Vol 131, No. 4, (1984).
It is further noted that spectrometers are well known in the art and typically comprise:
a. a means for receiving electromagnetic radiation;
b. a diffracting means;
c. a detector means.
In addition various reflective means can be included to direct entered electromagnetic radiation between entry point and detector. Many such spectrograph component patterns, (e.g. Czerny-Turner, Littrow, Bunsen, Monk-Gillieson, Ebert, Wadsworth, White Multiple Pass, Ebert-Fastie), are described in xe2x80x9cAnalytical Flame Spectroscopyxe2x80x9d, by Alkemade et al., Phillips Technical Library, Springer-Verlag, 11970). The Czerny-Turner system element configuration is identified as relevant as it is comprised of a diffracting means being positioned physically between a means for receiving electromagnetic radiation and a detector means on one side thereof, and first and second reflecting means on a second side thereof. In use electromagnetic radiation is caused to enter said means for receiving electromagnetic radiation and reflect from said first reflecting means, then interact with said diffracting means such that a diffracted spectrum of electromagnetic radiation is caused to reflect from said second reflecting means and enter said detector means. While Czerny-Turner spectrometers with first and second reflecting means which have focal lengths on the order of two-hundred-fifty t250) millimeters or more are known in the industry, similar spectrometers with focal lengths less than two-hundred-fifty (250) millimeters, (which can fit on a card which can be plugged into a computer slot), are typically subject to aberations entered into electromagnetic beams caused to enter thereto, because to fit physical components into the space available typically requires that electromagnetic beams be caused to approach first and second reflecting means at angles which are other than near normal, (and aberation effects become more pronounced where said incidence angle deviates from normal). Additionally, it is directly stated that no known Czerny-Turner type spectrometer, (particularly those with reflecting means that have focal lengths of less than two-hundred-fifty (250) millimeters), configure elements therein such that at least a part of detector means packaging is physically positioned in back of the grating present therein, (so that electromagnetic radiation reflecting from the second reflective means can not access said packaging), in order to allow an electromagnetic beam which is caused to enter into said Czerny-Turner type spectrometer to interact with the first and second reflective means at a near normal angle of incidence. This is particularly true where the first reflective means has a focal length which is less that that of the second reflective means.
Patents to Woollam et al. U.S. Pat. No. 5,373,359 and to Johns et al., U.S. Pat. No. 5,666,201, are disclosed as they are known and systems described therein include spectrometer systems. The 201 Patent describes an effective spectrometer system, in an ellipsometer setting, which spectrometer system intercepts and utilizes electromagnetic radiation present in multiple orders which arise from interaction with a grating dispersive means.
Continuing, while direct comparison of changes in spectrometer provided spectra, or individually selected wavelengths therein, over time, can provide first order insight as to when products of a silicon dioxide etch, for instance, is optimally complete, certain mathematical techniques are available which can increase detection sensitivity to etch end points. One such technique is termed xe2x80x9cPrincipal Component Analysisxe2x80x9d (PCA) and has as its basic focus the reduction of a large data set of correlated measurements to a smaller set of components which allow easier interpretation while retaining significant information content.
A very relevant mathematical technique which can be considered a subset of xe2x80x9cPrincipal Component Analysisxe2x80x9d (PCA), and which utilizes tracking of xe2x80x9cEigenvaluesxe2x80x9d, is known as xe2x80x9cEvolving Factor Analysisxe2x80x9d (EFA). Generally, said technique provides that a Matrix be formed which can initially consist of a single row (column) of pixel values which correspond to intensity magnitudes of a spectrum of electromagnetic radiation at selected wavelengths, (typical practice is to utilize a sequence of wavelengths which are separated by fixed steps and which serve to provide data matrix pixil values). Said (EFA) technique then provides that a number of Eigenvalues be evaluated by Matrix manipulation. (The number of Eigenvalues selected is typically called the xe2x80x9cRankxe2x80x9d of the Matrix and determination thereof is discussed more supra herein). This is followed by adding a second row of values which correspond to intensity magnitudes of a spectrum of electromagnetic radiation, typically at the same wavelengths as utilized in obtaining data for the first row (column), but at a progressed time. Then again, Eigenvalues are evaluated utilizing the expanded Matrix. This procedure is continued, and Plots of Eigenvalues so arrived at over a period of time, often show, in at least some of said Eigenvalue plots, observable, measurable, detectable relatively abrupt changes therein which correspond very closely to etch end points.
A paper which describes the Evolving Factor Analysis technique in more detail is titled, appropriately xe2x80x9cEvolving Factor Analysisxe2x80x9d, and is authored by Keller and Massart. Said paper appeared in Chemometrics and Intelligent Laboratory Systems, 12 (1992) pp. 209-224 and is incorporated hereinto by reference. Said paper describes application to an evolving data set [x] which is present in Matrix form, and considers that said data set is to be decomposed into a Matrix of Column Factors [C(Pxc3x97N)] and a Mattlx of Row Factors tA(Nxc3x97Q)], where xe2x80x9cNxe2x80x9d is the number of Eigenvalues, (or Matrix Rank), to be tracked in time, and can correspond to the number of factors, (e.g. etch products), in a system being investigated. In addition, an Error or Noise Matrix [E(Pxc3x97Q)], is also assumed to exist. In mathematical symbolism this can be represented as:
[X]=[C][A]+[E]
and more descriptively as: 
The Keller et al. paper proceeds to state that the fundamental idea of (EFA) is to follow the change or evolution of the xe2x80x9cRankxe2x80x9d of the Data Matrix Ox] as a function of xe2x80x9cOrdered Variablexe2x80x9d. This is accomplished by a xe2x80x9cPrincipal Component Analysisxe2x80x9d on an increasing Data Matrix. Starting from a first Row of data, (which can be assumed to correspond to intensity magnitudes at selected wavelengths in a spectrum of wavelengths), the Eigenvalues are successively calculated for all Sub-Matrices [Xi] which are formed by the first i=1 . . . P Rows, according to:
[X]=[S][L]+[E];
where [S(ixc3x97N)] is the scores Matrix and [L(Nxc3x97N)] is the matrix of what is termed xe2x80x9cloadingsxe2x80x9d by Keller et al. The primary and most important step in said procedure is described as the correct determination of the unknown number of Factors N. To do this all Q Eigenvectors of the Data Matrix are usually determined first, (i.e. the size of S is initially (ixc3x97Q) and that of L is (Qxc3x97Q)). For the first Sub-Matrix [Xi], which is built by the first Row of the Data, an Eigenvalue is calculated. A second Row of Data is then added and the process is repeated. This is continued until all Data Rows are present and Eigenvalues are calculated for the resulting progressively larger Matrices. Next, the various Eigenvalues (EV""s), or more typically logarithms thereof (Log(EV""s), are plotted against the Ordered Variable and continuous lines filled-in between discrete points for each Eigenvalue. Where said plots rise out of a Noise level, the Rank (N) is increased by One (1). The Keller et al. paper focused upon determining the presence of substances in a sample as made observable via use of, for example, chromatography/spectra, and the Rank, which, it is stated, is a direct indication of the number of substances present. In theory, the number of substances present is equal to the number of Eigenvalues, calculated as described, which rise above a Noise level in the plots of Eigenvalues vs. Ordered Variable.
Another paper by Keller and Massart, titled xe2x80x9cArtifacts in Evolving Factor Analysis Based Methods For Purity Control In Liquid Chromatography, With Diode Array Detectorxe2x80x9d, Anal. Chem. Acta, 263 (1992) is also identified for general purposes.
A variation on (EFA) is Evolving Windowed Factor Analysis (EWFA). The general approach is similar to that described for (EFA), but a constant dimension Data Matrix is utilized. That is, before any data processing is performed, some number of scans are obtained, which number is sufficient to fill all rows (columns) of a selected dimension Matrix. The evolving aspect of this approach is that as new scans are entered to an entry row (column) of said selected dimension Matrix, a previously entered row (column) is forced out the other side of the xe2x80x9cMatrix Stackxe2x80x9d. The effect is that only the most recent scans are then represented in the Matrix. In addition, it is possible to modify the approach described by Keller et al. so that it is not Eigenvalues which are plotted, but rather members of a Diagonal Matrix [S] which when multiplied with another Matrix, constitutes an Eigenvalue Matrix. To elaborate, a Single Valued Decomposition (SVD) of a Data Matrix can be performed in which an Eigenvalue analysis of a non-square Matrix provides an Eigenvector (VT) and two additional Matrices xe2x80x9cUxe2x80x9d and xe2x80x9cSxe2x80x9d which are related as:
X=USVT,
where the product US is a Matrix of Eigenvalues of X, and where [S] is a Diagonal Matrix:   S  =      [                                        S            1                                    0                          0                          0                          0                          0                          0                          0                          …                                      0                                      S            2                                    0                          0                          0                          0                          0                          0                          …                                      0                          0                                      S            3                                    0                          0                          0                          0                          0                          …                                      0                          0                          0                                      S            N                                    0                          0                          0                                      0            M                                    …                                      …                          …                          …                          …                          …                          …                          …                          …                                      xe2x80x83                                ]  
(which Matrix, it is noted, need not be symmetrical. N is determined by the number of spectra in the matrix, and M by the number of pixils in each spectrum).
Practice of said Data Matrix Decomposition approach requires that after adding a new scan to a Data Matrix, (which pushes a previous value out the opposite side), Mathematical Decomposition of the resulting Data Matrix be performed with the result being that a new Diagonal Matrix [S] is formed. The values of the members of said Diagonal Matrix [S] can then be plotted as a function of time. Relatively abrupt changes in certain of said Diagonal Matrix [S] member plots can be indicative of such factors as application of Plasma forming energy, entry of etching gas, etching system perturbations, and, most importantly, etch end points. The present invention can utilize plots of Eigenvalues or members of said Diagonal Matrix [S], but it is noted that the latter has proven to provide convenient and fully satisfactory results. It is noted that a benefit of the (EWFA) approach is that the dimension of the Diagonal Matrix [S] is constant and is set by user selected Data Matrix Rank.
For general information, Decomposition of Data Matrices is described xe2x80x9cMatrix Computationsxe2x80x9d by Golub and Van Loan, published by John Hopkins Press, 1989; and in xe2x80x9cSolving Least Square Problemsxe2x80x9d by Lawson and Hanson, published by Prentice Hall in 1974; and in Handbook of Automatic Computationsxe2x80x9d by Wilkinson and Reinsch. Said references are all incorporated by reference hereinto.
An unpublished reference which describes the (EWFA) approach is a Ph.D. thesis to be presented by Michael Splichal at the University of New Mexico in 1996 titled xe2x80x9cAn Integrated System for Real-Time Feedback and Control of a Plasma Reactor Based upon Optical Emission Sensors and Chemometric Analysisxe2x80x9d. Said thesis is incorporated hereinto by reference.
The present invention is found in a specific application of a modified (EFWA) approach, preferrably in conjunction with use of a unique present invention spectrometer design.
A search of Patents has turned-up very little in addition to the known spectrometer related 201 and 359 Patents already cited infra herein. However, a U.S. Pat. No. 5,658,423 to Angell et al. was identified and is disclosed as it refers to developing a model of the principal components of spectral data obtained during an etch procedure. Further, a Patent to Rietman et al., U.S. Pat. No. 5,654,903 is disclosed as describes a method and intelligent apparatus for use in real-time monitoring of plasma etch procedures. U.S. Pat. No. 5,644,503 to Ito et al. is disclosed as it describes a mathematical analysis procedure in which eigenvalues are calculated. A Patent to Gifford, U.S. Pat. No. 5,347,460 is disclosed as it describes a mathematical analysis procedure for application to optical emission spectrographic data in semiconductor fabrication. A Patent to DuFault et al. U.S. Pat. No. 4,721,114 is disclosed as it describes a mathematical technique, albeit applied in cardiographic detection of P-Waves in ECG recordings. A U.S. Pat. No. 5,655,540 to Seegobin is also identified as it describes, in an ECG setting, use of an error reducing mathematical technique in which a normal data set is formed by averaging normal population data, and a subject data set is similarly determined and mean and standard deviation therefore are calculated for both data sets, and compared to provide results, in error reducing manners. This is mentioned as the present invention can utilize a similar approach wherein, on a per pixel basis, actual obtained etch end point data can have a corresponding mean xe2x80x9cnormalxe2x80x9d data set value subtracted therefrom, with said result of said subtraction being divided by the corresponding xe2x80x9cnormalxe2x80x9d data set standard deviation. The result can be utilized as a reduced data matrix to which is applied EWFA techniques to provide Diagonal factors [s]. This is an optional step which has been shown to remove false end-point indications in some cases.
A known Patent to Welch, U.S. Pat. No. 5,663,584, is also disclosed as it describes use of a plasma silicon dioxide etching procedure applied in fabrication of Schottky barrier based (MOS) devices, including inverting and non-inverting single devices which operate similar to multiple device Complimentary Metal Oxide Semiconductor Field Effect Transistor (CMOS) Systems.
As alluded to, the present invention has primary application in the area of detecting etch end points. And as will be described in the remaining Sections of this Disclosure, the method of Evolving Windowed Factor Analysis (EWFA), preferably as applied to emissive intensity spectra obtained utilizing a present invention spectrometer system, is preferably utilized in the present invention. As will become apparent, however, other spectrometer systems can be used in certain embodiments, and certain operational speed enhancement attributes of said present invention method, which are not disclosed or suggested by any reference known to the Inventors herein, serve to make the present invention particularly applicable to use in Real Time.
The present invention can be beneficially observed from three major viewpoints.
First, the present invention provides a spectrometer system which provides novel relative positioning of components therein.
Second, said present invention spectrometer system can be considered as utilized in combination with a method of multifactor analysis in both real time and as applied in retrospect to obtained stored data.
Third, said present invention can be considered to comprise application of a unique, high efficiency, high speed, xe2x80x9cMultifactorxe2x80x9d Evolving Windowed Factor Analysis (EWFA) method to determine semiconductor etch end points in real time, thereby enabling not only semiconductor fabrication process monitoring, but also process control.
In more detail, the present invention spectrometer system sequentially comprises:
a. at least one means for receiving electromagnetic radiation;
b. a first reflecting means with a focal length less than two-hundred-fifty (250) millimeters; at least one diffracting means;
c. at least one diffracting means;
d. a second reflecting means with a focal length less than two-hundred-fifty (250) millimeters; and
e. at least one detector means consisting of centrally located active detectors and laterally disposed packaging;
said diffracting means being mounted on a stage, (preferably rotatable), which is positioned physically between said means for receiving electromagnetic radiation and said detector means on one side thereof, and said first and second reflecting means on a second side thereof; such that, in use, electromagnetic radiation is caused to enter said means for receiving electromagnetic radiation and reflect from said first reflecting means, then interact with said diffracting means such that a diffracted spectrum of electromagnetic radiation is caused to reflect from said second reflecting means and enter said detector means, in which spectrometer system the first reflecting means has a focal length which is less than that of said second reflecting means and in which spectrometer system at least part of the detector means laterally disposed packaging is positioned behind said diffracting means in the sense that electromagnetic radiation reflecting from said second reflecting means is blocked direct access thereto by said diffracting means. It should be appreciated that positioning said detector means laterally disposed packaging behind the diffracting means allows positioning the active detectors laterally very near the grating, which is not possible if the detector means laterally disposed packaging is positioned laterally adjacent to the diffracting means. All known spectrometer systems which comprise reflecting means with a focal length less than two-hundred-fifty (250) millimeters position detector means laterally disposed with respect to the diffracting means and thus require that entering electromagnetic radiation interact with reflecting means therein at angles of incidence which significantly deviate from normal. As a nonlimiting example, the focal length of said first reflecting means can be in the range of fifty-eight (58) to sixty-two (62), (nominal (60)), millimeters, and the focal length of said second reflecting means can be in the range of seventy (70) to eighty (80), (nominal (75)), millimeters. However a focal length of said first reflecting means in the range of fifty (50) to one-hundred-twenty (120) millimeters, and a focal length of said second reflecting means in the range of sixty (60) to one-hundred-sixty (160) millimeters is well within the scope of the present invention.
It is further noted that said spectrometer system means for receiving electromagnetic radiation can comprise a slit with dimensions of between five (5) to thirty (30) microns by one-hundred (100) to two-thousand (2000) microns, with nominal values of seventeen (17) by one-thousand (1000) microns. Further, said means for receiving electromagnetic radiation can comprise a means for accepting a fiber optic.
In addition said spectrometer system typically further comprises a printed circuit board with plug means for effecting electrical contact to an expansion slot in a computer system, said spectrometer system being mounted to said printed circuit board via vibration absorbing and stress relieving means such as compliant rubber spacers.
Said spectrometer system detector means can comprise electrical contact pins suitable for mounting to integrated circuit sockets and to printed circuit boards, Said detector means is, however, preferably mounted by other than physical interconnection to said electrical contact pins, with said electrical contact pins being electrically accessed via stress relieving flexible means. And said spectrometer system can further comprise a computing means to which said detector means is electrically interconnected, such that in use signal(s) corresponding to detected electromagnetic radiation is/are input to said computing means by said detector means.
Said spectrometer system can further comprise a filter means placed prior to said detector means. Said filter means -can be comprised of at least one element which provides utility by serving to separate out the wavelengths of other than a first order produced by interaction of said electromagnetic radiation with said diffraction means, from wavelengths of a first order and allow only wavelengths of said first order to enter said detector means, or by serving to selectively attenuate certain high intensity signals. With respect to the later point, it can happen that a relatively large magnitude intensity peak is present at a certain wavelength in a spectrum of electromagnetic wavelengths. Where this is the case, to measure said large magnitude intensity can require that sensitivity of a measuring detector system be set such that lesser magnitude intensities are not measurable. Selectively attenuating said relatively large magnitude intensity can then enable simultaneous measurement of it, and said lesser magnitude intensities. Of course, the attenuation factor will typically be mathematically factored back in to provide accurate end result measurement of said relatively large magnitude intensity.
It is also noted that a preferred spectrometer system diffraction means is a grating, and that said detector means is preferably selected from the group consisting of charge coupled devices, charge injection devices, and photo diode arrays.
While better understood by reference to the Drawings, it is noted for emphasis, at this point, that the present invention spectrometer system positions the mirrors and grating therein so that an electromagnetic beam approaches them at very near to a normal angle of incidence. This is extremely important because the closer to a normal angle of incidence a beam of electromagnetic radiation makes to, for instance, a mirror, the less significant are aberration effects induced by said mirror, in a reflected beam of electromagnetic radiation. The importance of avoiding the aberration entering effects of mirrors in a spectrometer system is of critical importance where optimum wavelength resolving power is desired. For example, application of the present invention has specifically shown that resolution of wavelengths which, vary from one another by one to two tenths of a nanometer (1/10-2/10 nm) or so can be critical to precise detection of etch endpoints. For instance, it is the case that where said resolution is not possible, a detector might provide an output of xe2x80x9cXxe2x80x9d=xe2x80x9cY+Zxe2x80x9d, where xe2x80x9cYxe2x80x9d and xe2x80x9cZxe2x80x9d are output xe2x80x9cXxe2x80x9d constituents contributed by different wavelengths. It can happen that at an etch end point one of the constituents xe2x80x9cYxe2x80x9d and xe2x80x9cZxe2x80x9d can increase and another decrease, providing a relatively constant value for xe2x80x9cXxe2x80x9d. That is, an increased intensity in one component is offset by a decreased intensity in the other, leaving the resulting detector output xe2x80x9cXxe2x80x9d essentially constant. If, however, resolution of xe2x80x9cYxe2x80x9d and xe2x80x9cZxe2x80x9d can be achieved, and separate detector signals developed for each, then evidence of an end etch point is made much more evident. Again, the present invention spectrometer system positions the grating and mirrors present therein so that a beam of electromagnetic radiation interacting therewith in use, approaches the mirrors at closer to a normal angle of incidence than have previous compact spectrometer systems, and this enables provides greater wavelength resolving capability. The present spectrometer design enables greater etch end point sensitivity by providing better data for evolving windowed factor analysis. As better described supra herein, the changes in members of a diagonal matrix [S] or eigenvalues [US] of a data matrix=[X]=[U][S][VT], plotted against time, become sharper and are better indications of etch end points, where spectrometer provided data is of better quality, and greater spectrometer wavelength resolution capability interprets to higher quality data.
When combined with means for etching semiconductor substrates, said present invention spectrometer system can be considered to be a semiconductor etch end-point detecting system. It is noted that systems for etching semiconductor substrates typically comprise means for effecting plasma etching and consist of:
a. at least one vacuum chamber in which a semiconductor system to be etched is present during use;
b. at least one means for entering etching gas to said vacuum chamber;
c. at least one means for applying electrical energy to said etching gas;
d. at least one means for accessing electromagnetic radiation present in said vacuum chamber during a semiconductor etching process; and
e. at least one means for guiding said accessed electromagnetic radiation into said spectrometer system means for receiving a electromagnetic radiation.
(it is noted that typical commercial plasma etching systems are available from manufacturers such as Applied Materials (AMAT), LAM Research and Tokyo Electron (TEL)).
With the mayor aspects of the System of the present invention now described, attention is turned to various aspects of Method.
In its most basic sense, the method of the present invention involves identifying semiconductor etch end points in both real time, and in retrospect. One embodiment, which relies upon use of the present invention system described infra herein, comprises the step of:
A. providing a semiconductor etch end-point detecting system as described infra, and then chronologically repeatedly performing steps B. through F. in an evolving windowed factor analysis sequence until detecting semiconductor etch end point, said steps B. through F. being:
B. during a semiconductor etch procedure in a vacuum chamber of said system, obtaining a chronological sequence of electromagnetic radiation intensity vs. wavelength spectra from said spectrometer system detector means, said spectrometer system detector means being caused to access electromagnetic radiation present in said vacuum chamber during a semiconductor etching process;
C. selecting some number of electromagnetic radiation intensity vs. wavelength spectra from said chronological sequence of electromagnetic radiation intensity vs. wavelength spectra and forming them into a data matrix;
D. optionally selecting and deleting some set-off number of rows (columns) in said data matrix, typically other than first and last rows (columns);
E. by applying mathematical matrix decomposition techniques to said data matrix determining value(s) of at least one representative parameter(s), each said representative parameter(s) being selected from the group consisting of: (members of a diagonal matrix [S] and eigenvalues [US] where:
Data Matrix=[X]=[U][S][VT]);
F. detecting semiconductor etch end point based upon change in said repeatedly calculated at least one representative parameter value(s) resulting from said chronologically repeated performance of steps B. through F.
Said method of identifying semiconductor etch end points can further comprise at least one step selected from the group consisting of:
a. in conjunction with said step B. obtaining of a chronological sequence of electromagnetic radiation intensity vs. wavelength spectra from said detector means, the performing signal to noise ratio enhancing technique(s), such that said each of said electromagnetic radiation intensity vs. wavelength spectra in said chronological sequence thereof utilized to form said data matrix in step C. are composite electromagnetic radiation intensity vs. wavelength spectra with improved signal to noise ratios; and
b. in conjunction with said step C. the step of identifying critical wavelengths and deleting intensity values at other wavelengths in said spectra.
The most important application of the method of the present invention provides for identifying semiconductor etch end points in real time and can comprise chronologically repeatedly performing steps a. through f. in an evolving windowed factor analysis sequence until detecting semiconductor etch end point, said steps a. through f. being:
a. while performing a semiconductor etch procedure in a vacuum chamber, obtaining a chronological sequence of electromagnetic radiation intensity vs. wavelength spectra from a detector means, said detector means being positioned so as to receive electromagnetic radiation eminating from said vacuum chamber during said semiconductor etch procedure;
b. selecting a period of time and for each of a sequence of said selected time periods performing signal to noise ratio enhancing technique(s) to two or more electromagnetic radiation intensity vs. wavelength spectra obtained thereduring, with the result being a chronological sequence of composite spectra with improved signal to noise ratios;
c. selecting some number of composite spectra in said chronological sequence of composite spectra, and forming them into a data matrix consisting of a definite number of rows and columns;
d. optionally selecting and deleting some set-off number of rows (columns) in said data matrix;
e. by mathematical matrix decomposition techniques determining values for at least one representative parameter(s) selected from the group consisting of: (members of a diagonal matrix [S] and eigenvalues [US]), where
Data Matrix=[X]=[U][S][VT]);
xe2x80x83which representative parameter(s) monitor changes in said chronological sequence of composite spectra;
f. detecting semiconductor etch end point based upon change in said repeatedly calculated at least one representative parameter value(s) resulting from said chronologically repeated performance of steps a. through f.
Said method of identifying semiconductor etch end points in real time can further comprise the step of identifying critical wavelengths and deleting intensity values at other wavelengths in said composite spectra.
A somewhat different presentation of the present invention method of identifying semiconductor etch end points in real time comprises chronologically repeatedly performing steps a. through e. in an evolving windowed factor analysis sequence until detecting semiconductor etch end point, said steps a. through e. being:
a. while performing a semiconductor etch procedure in a vacuum chamber, obtaining a chronological sequence of electromagnetic radiation intensity vs. wavelength spectra from a detector means, said detector means being positioned so as to receive electromagnetic radiation eminating from said vacuum chamber during said semiconductor etch procedure;
b. selecting some number of electromagnetic radiation intensity vs. wavelengths spectra in said chronological sequence of electromagnetic radiation intensity vs. wavelengths spectra, and forming them into a data matrix consisting of a definite number of rows and columns;
c. selecting and deleting some set-off number of rows (columns) in said data matrix;
d. by mathematical matrix decomposition techniques determining values for at least one representative parameter(s) selected from the group consisting of: (members of a diagonal matrix [S] and eigenvalues [US]), where
Data Matrix=[X]=[U][S][VT]);
xe2x80x83which representative parameter(s) monitor changes in said chronological sequence of electromagnetic radiation intensity vs. wavelength spectra;
e. detecting semiconductor etch end point based upon change in said repeatedly calculated at least one representative parameter value(s) resulting from said chronologically repeated performance of steps a. through e.
Said present invention method of identifying semiconductor etch end points in real time can further comprise at least one step selected from the group consisting of:
a. in conjunction with said step b. obtaining of a chronological sequence of electromagnetic radiation intensity vs. wavelength spectra from said detector means, the performing signal to noise ratio enhancing technique(s), such that said each of said electromagnetic radiation intensity vs. wavelength spectra in said chronological sequence thereof utilized to form said data matrix are composite electromagnetic radiation intensity vs. wavelength spectra with improved signal to noise ratios; and
b. identifying critical wavelengths and deleting intensity values at other wavelengths in said obtained electromagnetic radiation intensity vs. wavelength spectra.
With the foregoing in mind, it is further noted that plasma etching systems often operate with a xe2x80x9crotating plasmaxe2x80x9d basis in which the position of the center of a plasma within an etching system varies during use. This variation is typically essentially cyclical around an effective central point in a plasma etching system, with a period on the order of two (2) to three (3) seconds. As a result, the intensity of Electromagnetic radiation detected by an essentially stationary position detector typically varies during an etching procedure in a cyclic manner with the same period, (e.g. on the order of two (2) to three (3) seconds), as the xe2x80x9crotating plasmaxe2x80x9d. Further, it is noted that known signal integrating electromagnetic wave detector systems can serve to process varying signals and provide a xe2x80x9csmoothedxe2x80x9d output. However, signal integrating electromagnetic wave detector systems, being biased by a finite voltage power source, demonstrate saturation effects where too long an integration period is selected. A preferred embodiment of the present invention recognizes both of these well known facts, and, to improve etch end point detection capability, provides that in a preferred embodiment a detector system integrate signals input thereto during an etching procedure over a series of time periods which are each a fraction, (eg. xc2xd, ⅓, xc2xc, xe2x85x9 etc.), of the plasma rotation period. This is combined with the addition of the results of said integrations obtained during a single plasma rotation period to provide a xe2x80x9cper plasma rotation periodxe2x80x9d output signal. In practice, the plasma rotation period is empirically determined, and the fraction of the plasma rotation period selected as an integration period is determined so as to avoid integration circuitry saturation effects, while still providing a measurable output signal in a range which is on the order of fifty (50%) to eighty (80%) percent of the integration circuitry saturation value. The described procedure has been found to improve the signal to noise ratio of the present invention, particularly where combined with elimination of wavelengths from a spectrum which are empirically observed to be xe2x80x9cnoisyxe2x80x9d, (ie. somewhat unpredictably vary over time).
It is also noted that certain groupings of wavelengths are particularly well suited to detection of etching end points of specific materials. This is because the plasma etching products vary with the substrate etched. For instance, where Silicon Dioxide (SiO2) is etched, the products include Si, SiF and SiF2, but where Silicon Nitride is etched, the products include SiN. Electromagnetic wavelengths emitted by different etch products are, of course, different. The present invention includes the use of Wavelength xe2x80x9cMaskxe2x80x9d which pass relevant sets of wavelengths to a detector, and not others. This result can be achieved by digital filtering techniques, for instance.
Further, while the experimental work supporting this invention has focused on application to etch end point detection, it is to be understood that the spectrometer and mathematical techniques are applicable to any procedure which provides varying content electromagnetic radiation at an input to said spectrometer system. For instance, plasma cleaning of a process chamber can be similarly monitored.
The present invention will be better understood by reference to the Detailed Description Section of this Disclosure, in conjunction with the accompanying Drawings.
It is therefore a purpose of the present invention to teach a spectrometer system which provides novel relative positioning of components therein.
It is another purpose of the present invention to teach utilization of said present invention spectrometer system in combination with a multifactor analysis method in both real time and as applied in retrospect to obtained stored data to enable detection of semiconductor etch end points.
It is yet another purpose of the present invention to teach a multifactor analysis method which can, in real time, be applied to determine semiconductor etch end points, and thereby enable semiconductor fabrication process control, said multifactor analysis method being characterized by selections from the group consisting of the use of bifrucated window matracies, and the use of mask selection of wavelength sets, and the compensation of plasma rotation effects by use of a fractional integration period combined with a full period representing results summation procedure.